Photo mechanical imaging, electroless and electro plating

In the early days of circuit boards, life was simple. You made a silk screen image in negative of your tracks, screen printed them onto copper clad laminate using an etch resist ink, cured the ink, etched the exposed copper off, stripped the ink and hey presto, basically your circuit board.

Then came the requirement for plated holes. What a life change. The resist first of all changed from an etch resist to a plating resist so the tracks could be selectively plated rather than plating the whole copper surface. The tracks became thinner to allow more interconnections for the designers, and the process became a whole lot more complex. For this discussion document, if we take the three processes in order like this.

Electroless copper plate. When the double sided copper laminate is drilled, it is obvious that there is no connection from one side to the other. To make this happen, firstly, a chemical copper is plated across the whole board. This is a self limiting process and once it has achieved about 2 to 3 microns on a hi build system, it stops, or slows so much it becomes negligible. It does plate very slightly on the surface copper but due to a pretreatment in the plating line is more attracted to the exposed substrate in the hole. Bingo, we have an interconnection from one side to the other. The problem is it is so thin it has no current carrying capacity. It needs to be thicker. We really only want it to be thicker in the areas we want to keep as plating the base copper all over would just increase out etching requirement. This is where we introduce:

Photo mechanical imaging. Once we have the thin plating down the hole, we need to beef it up, usually to 25 microns using electroplating but to prevent the copper going all over the panel, we create a negative image of the tracks in the copper in a substance known as dry film resist.

The first part of the process is to apply completely over the panel, a film not unlike sticky tape. This film is light sensitive so must be processed under yellow safe light. This film is applied using heat and pressure, so there are usually 2 heated rollers running together and held there by compressed air cylinders. This ensures all of the air is pressed out between the resist and the surface of the copper. This must be done in a clean environment. Although this is known as dry film resist, it is still tacky to the touch at this stage and can be scraped using a finger nail.

Now, a positive black and clear image of the tracks needs to be registered to the panel so the pads on the artwork line up with the hole pattern. There are numerous tooling systems to accurately do this. The sandwich of artworks and laminate is now placed in a print frame. Usually a glass bottom and a clear plastic film in a frame. A vacuum is then pulled in between the glass and the clear plastic film so the artworks are pressed right against the dry film resist. This must be totally intimate contact, no air gaps or the light will spread resulting in bad tracks.

Once the vacuum is pulled, the sandwich is exposed top and bottom to UV light which cures the dry film resist where the clear artwork is and protects the resist where the black photography is. Once this is done, the sandwich can be removed and dis assembled. The dry film resist is now hard to the touch where the clear film was and soft where the black bit was. This soft resist can now be dissolved in a special solution. What is left is an image of the tracks required in the dry film plating resist. These must now be plated up using:

Electrolytic plating. Electrolytic plating has been around for many tears, chrome, gold silver etc. These are decorative finishes.

Copper plating is carried out by attaching the panel to a conductive jig, putting it into a bath of sulphuric acid with copper dissolved in it, and between anodes on each side of the panel. A current is then passed in to the anode, goes through the electrolyte and onto the exposed copper of the panel depositing copper Copper however in this instance, is functional to carry current so must be controlled carefully. Electroplating is technically challenging due to the nature of the process. Electricity always takes the easiest route so the surface is a lot easier to access than the holes. To this end, the solution of sulphuric acid has additives added which ‘stick’ to the copper and inhibit plating. The jig is usually agitated back and forward. This results in the solution being pushed through the holes where it washes off the inhibitors resulting in the holes being easier to plate than the surface where they are stuck. This is how a good surface to hole plated ratio is achieved.

Once the copper has reached 25 microns, the process moves on to a similar process but using and etch resist like tin. Once this is completed, the etch plating resist is removed exposing the unwanted copper. The panel can now be passed through an etch machine. When this is done, the board is functional and just requires finishing processes.

This is an appreciation more than a definitive process specification. If you want to add to or query any part or if you want more details, drop me an email.

Next month, solder mask and legend, unless anyone can suggest something better to put under the eyeglass.